This invention relates to high thermal conductivity carbon fibers and a process for producing them.
Many devices in everyday use are required to disperse substantial amounts of heat in order to function effectively. Electronic devices, such as computer circuits, and mechanical devices, such as aircraft brakes, are two examples. Carbon fibers have long been recognized as excellent conductors of heat, but the drive toward miniaturization and the use of advanced composite materials, many of which do not conduct heat efficiently, require still better thermally conductive fibers.
There have been several approaches to improving the conductivity of carbon fibers. In one method, carbon fibers are given a post-graphitization annealing step. See commonly assigned U.S. Ser. No. 07/491,582. The annealing involves relatively mild heating after graphitization. A fiber having a thermal conductivity of 740 watts/mK is exemplified.
European Patent Application 0 372931, published Jun. 13, 1990, reports examples achieving electrical resistivities as low as 1.15 micro ohm meters which is believed to correspond to a thermal conductivity of about 910 watts/mK. The European Application describes extreme measures to maximize density as the route to achieving maximum conductivities. To maximize the density, the fibers are heated to high temperatures, in the range of 3200 to 3521 degrees Celsius, for very long periods, from one to two hours, an expensive operation.
This invention also focuses on obtaining fibers with high densities and high conductivities. However, rather than trying to densify the fiber after it is formed, this invention provides a process in which the texture and microstructure of the fiber is controlled during the formation of the fiber to result in high density, high conductivity fibers without requiring extreme conditions and lengthy times during graphitization. The high densities and high conductivities of the fibers produced by the process of this invention are achieved by making the texture of the fibers as radial in character as possible, and by forming a fiber with a microstructure that is as highly susceptible to forming aligned graphitic planes as possible. Thus the process of this invention achieves high densities and conductivities not by forcing unaligned graphite planes together after formation, but by aligning the planes so they fit together compactly from the outset.
It is well recognized in the art that radial texture in carbon fibers leads to axial cracking, sometimes called "pacman" formation. However the prior art that has dealt with this phenomenon has been devoted to the minimization or total avoidance of the pacman or axial cracks. The cracking was viewed as a barrier to achieving optimum strength in carbon fibers because the regions exhibiting such cracks were thought to be where tensile failures tended to occur.
There are many prior art teachings relating to minimization of the formation of pacman cracks. See, for example, commonly assigned application EP 0383339 which teaches a particular configuration for disruption of the flow of pitch at the entry to the spinneret as a means of avoiding radial structure and axial cracks. Another reference is Riggs and Redick, U.S. Pat. No. 4,567,811, also commonly assigned. This patent does not specifically mention the formation of axial cracks due to radial texture in carbon fibers, but teaches spinneret geometries selected for the purpose of optimizing fiber strength While there have been many who have attempted to avoid radial texture and resulting crack formation, there has been no teaching that recognized that radial crack formation was really a process through which the fiber density was being increased, and that this could lead to increased thermal conductivity. Further, while many references purport to teach how to reliably avoid formation of radial structure in carbon fibers, no reference has taught how to achieve radial structure so completely that axial cracks form along nearly the entire length of any fiber and so that such cracks which do form are as large as possible.
This invention provides carbon fibers of high densities and high conductivities. These properties are the result of radial fiber texture leading to densification of the fiber by formation of axial cracks, and are also the result of highly aligned and therefore closely packed microstructure of the fibers.
As used in this application, the term conductivity refers to both electrical and thermal conductivity, and it is believed that these properties correlate, so that if an electrical conductivity were specified, a corresponding value of thermal conductivity could be estimated. Similarly, electrical conductivity is the inverse of electrical resistivity, so any resistivity value has its unique electrical conductivity counterpart. While all of these variables are interrelated, and while increased conductivity is the object of this invention, electrical resistivity is the easiest to measure, and therefore, data given and parameters described herein will be in terms of electrical resistivity. Where thermal conductivity values are reported, these will be estimated figures based on electrical resistivity measurements.